The compression ratio (CR) of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). Generally, the higher the compression ratio, the higher the thermal efficiency and fuel economy of the internal combustion engine. Variable Compression Ratio (VCR) engines have been developed wherein the compression ratio of each cylinder can be varied between a higher and a lower setting to improve engine performance. For example, the higher compression ratio setting may be used during knock-free conditions to take advantage of the high thermal efficiency while the lower compression ratio setting may be used during knock prone conditions. In the VCR engines, a linkage or other mechanism (e.g., an eccentric) may be coupled to the piston of each cylinder to mechanically vary the compression ratio between the higher and lower settings.
One example of a VCR engine is shown by Caswell at U.S. Pat. No. 4,469,055. Therein, during engine running, the CR of the engine is adjusted based on engine operating conditions. For example, the CR may be optimized for engine fuel efficiency or engine performance, or both. The CR calibration, that is the CR commanded as a function of engine speed and load, may be calibrated based on a prototype engine.
However, the inventors herein have identified potential issues with such systems. As one example, the adjustment of the CR during engine operation requires the actual CR to be known accurately. However, each engine may have a slightly different compression ratio (CR) in each cylinder, due to manufacturing tolerances. In a VCR engine, each component of the VCR mechanism may have manufacturing tolerances leading to significant part-to-part variation, in addition to the normal variation on non-VCR engines. As a result, calibration of a parameter affecting engine dilution, such as an exhaust gas recirculation (EGR) calibration or a variable cam timing (VCT) calibration that is based on CR data from the prototype engine may not be optimal for a production engine. The resulting higher cylinder-to-cylinder variation in CR can produce unstable combustion on those cylinders which have a lower-than-average CR, leading to higher noise, vibration, and harshness (NVH), higher risk of misfire, and lower efficiency on those cylinders. Use of premium manufacturing methods and/or “select fit” parts can be used to control or account for CR differences between cylinders, but such approaches add significant cost.
In one example, the above issues may be at least partly addressed by a method comprising: calibrating a compression ratio schedule of a variable compression ratio engine based on each of fuel flow and peak torque of each cylinder at each compression ratio setting of the engine; and adjusting exhaust gas recirculation (EGR) flow to the engine in accordance with an updated EGR calibration schedule based on the compression ratio schedule calibration. In this way, dilution optimization of a VCR engine is improved.
As one example, the actual CR of each cylinder of a VCR engine may be quantified as a function of each VCR mechanism setting. For example, the CR of each engine cylinder may be quantified at each of a plurality of compression ratio settings. In one example, the CR of each cylinder may be quantified based on a learned fuel flow and IMEP of each cylinder at each CR setting of the VCR engine. Then, a lowest of the quantified compression ratios may be identified. A nominal EGR or VCT schedule may then be updated based on the identified lowest CR of all the cylinders. The nominal EGR/VCT schedule may be created based on Mapping data from a prototype engine. During conditions when the engine load is higher than a threshold load, where engine dilution requirements are lower, engine dilution may be provided in accordance with the nominal EGR/VCT schedule. However, during conditions when the engine load is lower than the threshold load, where engine dilution requirements are higher, engine dilution may be provided in accordance with the updated EGR/VCT schedule. Specifically, as the lowest CR decreases, the engine dilution provided may be decreased, such as by reducing an opening of an EGR valve or by advancing one or more of intake valve closing timing and exhaust valve closing timing (thereby reducing positive valve overlap). In one example, the actual CR may be significantly lower than the commanded CR due to VCR mechanism degradation, or due to entry conditions for the optimum CR not being met (such as temperature, oil pressure, and current limit conditions not being met). By adjusting the EGR/VCT schedule during such conditions, combustion stability and engine performance can be improved while compensating for the lack of the optimum CR.
In this way, the efficiency of a VCR engine may be improved by better adjusting engine dilution in view of actual cylinder-to-cylinder variations in compression ratio. By learning fuel flow and IMEP of all cylinders as a function of each CR setting of the VCR engine, CR variations of the actual engine may be learned, instead of relying on a prototype engine which may be significantly different from the given engine. Further, the EGR and VCT schedule of the VCR engine can be calibrated without relying on expensive manufacturing methods and/or components. The technical effect of modifying a nominal EGR/VCT schedule of the VCR engine based on a lowest mapped CR of all the engine cylinders is that the efficiency of a VCR engine may be increased by allowing a base EGR/VCT calibration to operate closer to the stability limit while still protecting for engines with relatively low CR on one or more cylinders (due to manufacturing variations). By modifying the EGR and VCT schedule of the engine based on the actual CR mapping, dilution control at low engine loads is improved, allowing for improved combustion stability and reduced NVH. Overall, engine performance and fuel efficiency of a VCR engine can be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.